1. Field of the Invention
The present invention relates to the field of turbomachines. It concerns a compressor, in particular for gas turbines, including
(a) a rotor shaft which can be rotated about a compressor center line and which has a plurality of rotor blades fastened at its periphery; PA1 (b) a compressor casing surrounding the rotor shaft in the region of the rotor blades; PA1 (c) an inlet casing surrounding the rotor shaft at the inlet end of the compressor and having an outer shell and an inner shell between which is formed an inlet space for the air to be compressed, which inlet space is in connection with the surroundings at one end by means of an air inlet provided with an inlet filter and merges at the other end into an induction duct equipped with adjustable or fixed inlet guide vanes, the outer shell adjoining the compressor casing; and PA1 (d) a seal casing on the end of the inner shell facing toward the rotor blades, which seal casing, located at the periphery of the rotor shaft, seals the induction duct against the surroundings and includes a sealing air chamber from which sealing air can emerge into the annular gap between the seal casing and the rotor shaft. PA1 (e) the seal chamber is in connection with the inlet space by means of at least one sealing air passage. PA1 (a) a plurality of sealing ribs are arranged one behind the other in the direction of the compressor center line in the annular gap between the seal casing and the rotor shaft, which sealing ribs, starting alternately from the seal casing and the rotor shaft, protrude into the annular gap and define a radial clearance by their distance from the opposite side; and PA1 (b) the sealing ribs are subdivided into two groups and the sealing air flows out of the sealing air chamber into the annular gap through a sealing air opening arranged between the two groups.
Such a compressor is known, for example, from the article by J. P. Smed and H. Saeki, A NEW DESIGN FOR A COMPRESSOR INLET CASING ATMOSPHERIC VENT SYSTEM, ASME Cogen-Turbo, IGTI-Vol. 7, pp. 535-537 (ASME 1992).
2. Discussion of Background
In compressors, such as are used as part of a gas turbine, measures must be taken in order to seal spaces with different pressures on the rotating rotor shaft against one another during operation so that the efficiency of the compressor remains high and so that faults--such as can be initiated by lubricating oil from the bearings entering the compressor duct--are reliably avoided.
One possible type of seal is sealing with air under pressure, such as is described in US-A-3,031,132 for the gas turbine of an aircraft. In this, the rotor shaft is annularly surrounded at the position to be sealed by a seal chamber accommodated in a corresponding seal casing. The sealing air under pressure can emerge from the seal chamber into the annular gap between the seal casing and the rotor shaft and, by this means, limit or completely prevent the penetration of undesirable media into the annular gap. In this arrangement, the compressed air is generally tapped from a pressure stage, or optionally a plurality of pressure stages, of the compressor and is fed into the seal chamber by means of a suitable valve circuit and control system.
Such a compressed air seal can be arranged at various positions on the compressor. In the US patent cited, the seal casing--which can, simultaneously, also undertake cooling tasks--is arranged near the high-pressure end shaft bearing of the compressor. In the publication mentioned at the beginning, a compressor is described (see FIGS. 1 and 2 in that publication) in which the seal is arranged at the inlet end journal bearing where the rotor shaft emerges and the compressor casing merges into the inlet casing. This is principally intended to prevent unfiltered and possibly oil-contaminated external air from being forced into the inlet end, low-pressure part of the compressor via the bearings and mixing with the compressor air.
Where the compressor is operated with fixed or slightly altering parameters, the requirements for the compressed air sealing remain within tolerable limits. This solution becomes impracticable, however, if adjustable inlet guide vanes permitting the inlet air-flow to be throttled to a substantial extent at part-load are provided at the inlet to the compressor. In the case of severe throttling, the air pressure in the compressor reaches the atmospheric level after approximately the third compression stage at the earliest, and it is therefore necessary to take the compressed air from the fifth stage or later. If, on the other hand, no throttling occurs, the temperature and pressure of the compressed air taken from the fifth stage or later are, at 180.degree. C. and 4 bar, too high so that a switch-over valve is necessary for tapping the compressed air from a cooler stage.
In the publication mentioned at the beginning (Smed and Saeki), a seal operating with air has already been proposed which can operate without external inspection and control circuits (FIG. 4). This is achieved by admitting cooling air at atmospheric pressure through a passage to the seal chamber which is intended to separate the induction end of the compressor, which is subjected to a vacuum, from the bearings, which are likewise subject to a vacuum but to a smaller vacuum.
This known solution, however, introduces one main problem. The inlet-end cooling air has not generally been subjected to any special filtering so that here again, impurities can be introduced into the compressor duct (via the sealing air).